A conventional multi-carrier CDMA communication apparatus will now be explained. As a communication apparatus in a mobile communication system which adopts the multiple access scheme, using the multi-carrier CDMA method, there can be mentioned for example the one described in the literatures “Performance comparisons of coherent SC/DS-CDMA, MC/DS-CDMA, MC-CDMA on down-link broadband radio packet transmission”, The Institute of Electronics, Information and Communication Engineers, Technical Report IEICE RCS99-130 p. 63–70, October 1999, and “Overview of Multi-carrier CDMA”, IEEE Communications Magazine, p. 126–133, December 1997.
The construction and operation of the conventional multi-carrier CDMA communication apparatus will now be explained with reference to the drawings. FIG. 34 is a diagram which shows the construction of a conventional multi-carrier CDMA transmitting apparatus (“transmitter”), and FIG. 35 is a diagram which shows the construction of a conventional multi-carrier CDMA receiving apparatus (“receiver”).
In FIG. 34, reference symbol 501 denotes a convolutional coder, 502 denotes an interleaver, 503 denotes a serial/parallel conversion section (hereinafter, referred to as S/P), 510a, 510b and 510c respectively denote first, second, and the Nscg-th sub-carrier group modulation processing sections, 511 denotes a frame creation section, 512 denotes a copy section, 513 denotes an information modulation section, 514 denotes a frequency spreading section, 504a, 504b and 504c denote multiplexing sections, 505 denotes an inverse Fourier transform section, 506 denotes a guard interval (GI) adding section, 507 denotes a frequency transform section and 508 denotes an antenna.
On the other hand, in FIG. 35, reference symbol 601 denotes a frequency transform section, 602 denotes a frequency transform section, 603 denotes a guard interval (GI) removal section, 604 denotes a Fourier transform section, 610a, 610b and 610c respectively denote first, second, and the Nscg-th sub-carrier group demodulation processing sections, 611 denotes a frequency despreading section, 612 is a synchronization detector, 613 denotes a combining section, 605 denotes a parallel/serial conversion section (hereinafter referred to as P/S), 606 denotes a deinterleaver, and 607 denotes a Viterbi decoder.
FIG. 36 is a diagram which shows the format of a transmission slot for each sub-carrier. The transmission slot comprises a pilot symbol portion (known sequence) and a data portion.
FIG. 37 is a diagram which shows one example of impulse response of a frequency selective fading transmission line. For example, in the mobile communication system, radio wave reflects, diffracts and scatters due to surrounding buildings and geographical features, and incoming waves (multi-path waves) through a plurality of transmission lines interfere with each other, and hence an impulse response of the frequency selective fading transmission line occurs.
The operation of the conventional multi-carrier CDMA communication apparatus will be explained with reference to FIG. 34 and FIG. 35. It is assumed here that data transfer is performed between a base station and a plurality of terminals. At first, the operation of the transmitter will be explained.
For example, the convolutional coder 501 having received transmission data for an optional terminal generates coded data in accordance with a predetermined code rate. This coded data is written in the vertical direction in the interleaver 502 comprising a block, for example, having a longitudinal size of Nr (predetermined integer) and a lateral size of Nc (predetermined integer), and read out in the lateral direction. That is, the interleaver 502 outputs the rearranged signal as coded data.
The S/P 503 having received the coded data converts the data to parallel data for the number of Nscg (predetermined integer), and outputs the converted output to the sub-carrier group modulation processing sections 510a, 510b, . . . , and 510c, respectively. Since the same signal processing is performed in the first to the Nscg-th sub-carrier group modulation processing sections which perform modulation processing for each sub-carrier group, the operation of the first sub-carrier group modulation processing section 510a will be explained here, and explanation for other sub-carrier group modulation processing sections is omitted.
The sub-carrier group modulation processing section 510a receives the first data sequence in the parallel output from the S/P 503. The frame creation section 511 first divides the data sequence into a unit of Ndata, and adds the known sequence (pilot symbol) at the top thereof, to thereby generate the data frame of a sub-carrier group (1). The copy section 512 copies the received data frame by the number of a predetermined sub-carrier number Nsub, to generate data frames for sub-carriers (1, 1) to (1, Nsub). The information modulation section 513 executes QPSK modulation individually with respect to the data frames by the received number of sub-carriers to thereby generate modulation signals for the sub-carriers (1, 1) to (1, Nsub). The frequency spreading section 514 performs frequency spreading for each terminal or for each other channel to be transmitted, using modulation signals by the received number of sub-carriers and frequency spreading codes orthogonal to each other which has been given beforehand. This frequency spreading is realized by multiplying the modulation signals of the received number of sub-carriers by frequency spreading codes C (1, 1) to C(1, Nsub) ach code is expressed by ±1). As the frequency spreading code, a Walsch code, being an orthogonal code, is normally used.
The multiplexing section 504a generates a multiplexing signal by multiplexing similar signals by the number of sub-carriers from other users, with respect to the received signals by the number of sub-carriers after the frequency spreading.
The inverse Fourier transform section 505 uses sub-carrier signals of the number of Nscg×Nsub obtained by the multiplexing sections 504a, 504b and 504c, to perform inverse Fourier transform processing.
The guard interval adding section 506 copies the rear part of the symbol in the signal after inverse Fourier transform by the time τGI, and sticks the copied part to the top of the symbol. FIG. 38 is a diagram which shows the processing in the guard interval adding section 506. τGI is normally set so as to become larger than the delayed wave expanse τd on the transmission line shown in FIG. 37.
Finally, the frequency transform section 507 multiplies the signal after adding the guard interval by a carrier wave signal in the output of a frequency oscillator (not shown), and executes bandwidth limiting, using a band-pass filter (not shown), to thereby generate a transmission signal. The transmission signal is then output to the transmission line via the antenna 508. FIG. 39 is a diagram which shows the transmission signal expressed on a frequency axis.
On the other hand, the receiver receives the transmission signal affected by the frequency selective fading or the like, via the antenna 601. The frequency transform section 602 performs bandwidth limiting by means of a band-pass filter (not shown) with respect to the input signal, and then multiplies the signal after performing the bandwidth limiting by a signal synchronous to the carrier wave frequency output by the frequency synthesizer (not shown). The multiplied signal is subjected to wave filtering by a low-pass filter (not shown) so that only a low frequency component is output as a signal after frequency translation.
The guard interval (GI) removal section 603 outputs a signal in which the guard interval is removed and each symbol is continuously connected to each other. The Fourier transform section 604 having received the signal in which the guard interval has been removed performs the Fourier transform processing, to thereby output sub-carrier signals by the number of Nscg×Nsub. Each sub-carrier signal is transmitted to the first, the second, . . . , and the Nscg-th sub-carrier group modulation processing sections 610a, 610b and 610c, respectively, in order to perform demodulation processing for each sub-carrier group. In the first, the second and the Nscg-th sub-carrier group demodulation processing sections 610a, 610b and 610c, the same signal processing is performed. Therefore, only the operation of the first sub-carrier group demodulation processing section 610a will be explained herein, and explanation for the other sub-carrier group demodulation processing sections is omitted.
The sub-carrier group demodulation processing section 610a receives the first sub-carrier signals by the number of Nsub, and the frequency despreading section 611 multiplies the sub-carrier signals by the number of Nsub by an individually allocated spreading code, to thereby perform inverse spreading.
The synchronization detector 612 which has received each sub-carrier signal after frequency despreading uses the known sequence symbol added for each frame to estimate the transmission line to thereby perform synchronization detection. That is to say, the synchronization detector 612 synchronously adds the known sequence symbols by the number of Npilot in the frame, to thereby calculate a transmission line estimate value for each sub-carrier. The synchronization detector 612 then calculates a complex conjugate and an absolute value of the calculation results, and divides the complex conjugate by the absolute value, to thereby extract a phase component for each sub-carrier. Finally, the synchronization detector 612 multiplies the sub-carrier signal after the frequency despreading by the phase component for each sub-carrier, to perform synchronization detection.
The combining section 613 adds all the received sub-carrier signals after the synchronization detection to calculate the first sub-carrier group signal.
The P/S 605 receives the sub-carrier group signals from all the sub-carrier group demodulation processing sections, and converts these signals to serial signals. The serial signals are written in the lateral direction in the deinterleaver 606 which has blocks having a longitudinal size of Nr (predetermined integer) and a lateral size of Nc (predetermined integer), and read out in the longitudinal direction.
Finally, the Viterbi decoder 607 performs known Viterbi decoding with respect to the received signal after rearrangement.
As described above, in the conventional multi-carrier CDMA communication apparatus, even if the amplitude and the phase of the receiving wave is affected by the frequency selective fading which varies at random, an excellent bit error rate characteristic is obtained by setting the guard interval so that the expanse of the delayed wave is fitted therein and further by allocating the frequency spreading code for each user or for each channel.
In the conventional multi-carrier CDMA communication apparatus, however, the expanse of the delayed wave is large depending on the situation of the transmission line, and sometimes the expanse of the delayed wave may not be fitted in the guard interval. In such an instance, there is a problem in that the apparatus is affected by the frequency selective fading, thereby the influence of interference increases in the symbol, and an excellent bit error rate characteristic cannot be obtained.
There is another problem in that when the level of the delayed wave is larger than the preceding wave, the delayed wave is not utilized for increasing the quality of the input signal (for example, a signal power to interference power ratio (SIR)).
In the multi-media mobile communication, it is considered to be necessary to change the information rate adaptively, depending on the application to be handled. In other words, when the frequency spreading code is allocated for each user or for each channel to be used, it is necessary to allocate a channel depending on the information transmission rate. In the conventional multi-carrier CDMA communication apparatus, however, it is necessary that the frequency spreading codes are orthogonal to each other, and since the frequency spreading rate is fixed, there is a problem in that the information rate cannot be changed adaptively.
When hand-over is performed between sectors and between cells, so that communication is not broken off at the time of reception by a mobile station (terminal), it is necessary to change the frequency to be used on the transmission line, thereby causing a problem in that the frequency efficiency decreases.
Moreover, transmission power control is necessary on the base station side, in order to solve a problem in that the reception quality of the mobile station must be kept constant regardless of the distance from the base station. In the land mobile communication, however, the degree of influence by the frequency selective fading differs largely for each sub-carrier. Therefore, with the conventional method in which the input signal power is kept constant in the mobile station, there is a problem in that the input signal quality cannot be kept constant, while suppressing the influence with respect to other mobile stations (terminals).